Mechanisms of a number of human diseases have been elucidated. The recombinant DNA techniques and the techniques for transferring a gene into cells have rapidly progressed. Under these circumstances, protocols for somatic gene therapies for treating severe genetic diseases have been recently developed. More recently, attempts have been made to apply the gene therapy not only to treatment of the genetic diseases but also to treatment of viral infections such as AIDS and cancers.
Most of the gene transfer trials in humans approved by Food and Drug Administration (FDA) to date are ones in which a gene is transferred into cells with the use of a recombinant retrovirus vector. Since the retrovirus vector efficiently transfers the foreign gene of interest into cells and stably integrates the gene into their chromosomal DNAs, it is a preferable means of gene transfer particularly for the gene therapy in which a long-term gene expression is desired. This vector has been subjected to various modifications so as not to have a bad influence on the organism with the transferred gene.
For example, the replication function of the vector is deleted such that the vector used for the gene transfer does not replicate in the cells while repeating unlimited infection (gene transfer). Since such a vector (a replication-deficient retrovirus vector) cannot replicate by itself, a retrovirus in which the vector is encapsidated in a virus particle is generally prepared by using retrovirus producer cells (packaging cells).
Bone marrow cells are the preferable target cells for the somatic gene therapy since they can be handled in vitro and contain hematopoietic stem cells that are capable of self-replicating. It has been demonstrated that umbilical cord blood also contains a number of progenitor cells including hematopoietic stem cells. By performing the gene therapy in which a gene is transferred into such target cells, which are then transplanted into an organism, the transferred gene can be expressed in blood cells for a long period, and a disease can be healed for life.
However, the hematopoietic stem cell is one of the cells into which a gene is not readily transferred with high efficiency, despite of the studies by many groups. To date, the most efficient protocol for gene transfer with respect to hematopoietic stem cells from mice or other animals have been the method in which the hematopoietic stem cells are co-cultured with retrovirus producer cells. However, under the apprehension about safety, gene transfer in cell-free system with a lower risk of contamination of the retrovirus producer cells has been desired for clinical gene therapy methods for humans. Unfortunately, it is not easy to efficiently transfer a gene into hematopoietic stem cells without co-culturing with retrovirus producer cells.
Recently, it was reported that fibronectin, which is a component of the extracellular matrix, or a fragment thereof alone elevates the gene transfer efficiency into cells by a retrovirus (J. Clin. Invest., 93:1451-1457 (1994); Blood, 88:855-862 (1996)). Also, it has been demonstrated that a fibronectin fragment produced by genetic engineering technique has similar properties and can be utilized to efficiently transfer a foreign gene into hematopoietic stem cells (WO 95/26200).
Furthermore, it is disclosed in WO 97/18318 that a functional substance other than fibronectin (such as fibroblast growth factor, collagen etc.) elevates the gene transfer efficiency and that similar increase in the gene transfer efficiency is observed when a mixture of a functional substance having a retrovirus-binding activity and another functional substance having target cell-binding activity is used.
It is believed that the increase in gene transfer efficiency caused by these functional substances is due to the increase in chance of interaction between the retrovirus and the target cells which are co-localized with the aid of the substances.